This invention relates to fuel cell components and fabrication methods and, in particular, to supports for fuel cell anode electrodes and methods of making same.
FIG. 1 shows schematically a fuel cell assembly 1 of a molten carbonate fuel cell with direct internal gas reforming. As shown, an anode electrode 2 and a cathode electrode 3 are in direct contact with the an electrolyte matrix or tile 4. An anode support 5 abuts the anode electrode 2 and is followed by a corrugated anode current collector 6 which, in turn, is followed by a bipolar plate 7. Reforming catalyst 11 is housed in the anode-side fuel flow compartment or spaces defined between the anode current collector 6 and the bipolar plate 7.
On the cathode side, a cathode current collector 8 follows the cathode electrode 3. The current collector 8 is followed by a bipolar plate 9.
In the current state-of-the-art, anode electrode 2 is comprised of a porous, high surface area Ni alloy (see, for example U.S. Pat. No. 4,999,155), such as Nixe2x80x94Al (see, for example U.S. Pat. No. 4,659,379) or Nixe2x80x94Cr (see, for example U.S. Pat. No. 4,714,586), with Al and Cr as stabilizing agents to enhance high-temperature mechanical strength and prevent excessive anode sintering at cell temperatures of 500 to 700xc2x0 C. The anode 2 is partially filled with liquid electrolyte, and provides a catalytic surface for the three-phase (gas-electrolyte-electrode) reactions. Additionally, the anode is a thin layer (xcx9c10 mils) of particle assembly with a porosity near 50%, fabricated by tape casting.
In U.S. Pat. No. 5,558,948, a baseline anode fabrication method is disclosed with the objectives of in-situ anode electrode sintering and oxidizing, and in-situ electrolyte filling of the sintered and oxidized component. The patent also describes the anode support member 5 being formed from a metallic plate having a plurality of through openings for gas diffusion into the associated supported anode electrode 2.
As described in the ""948 patent, when assembled into the cell assembly 1, the anode electrode 2 is a green tape composed of metal powders and organic binders. During fuel cell conditioning, the binders and additives are usually removed below 400xc2x0 C., and the anode electrode powder bed itself virtually has no strength before in-situ sintering. The sintering usually only occurs at above 500xc2x0 C., strengthening the anode structure.
The above initial fragile condition of the anode electrode 2 necessitates the use of the anode support member 5 to provide and maintain the integrity of the porous anode structure prior to sintering. Furthermore, during long-term fuel cell operation, the anode electrode may deform under concentrated compressive forces carried to the electrode through the corrugated anode current collector 6. The anode support member 5 helps to redistribute and even out the compressive forces. In order to function as a support, the member 5 must have sufficient long-term creep strength and stiffness at fuel cell operation temperature.
One of the well-known catalyst decay mechanisms in a fuel cell is the electrolyte intrusion in each cell assembly 1 from the anode electrode 2. During the operation of the fuel cell, each anode electrode 2 is filled (5-50% of its void volume) with electrolyte. Thermodynamically, this electrolyte tends to wet the associated anode support 5 and the anode current collector 6, reaching the reforming catalyst 11. Once sufficient electrolyte reaches the catalyst 11, the catalyst is poisoned, and is no longer able to perform the hydrocarbon catalytic reforming function to generate sufficient hydrogen fuel for the anode reaction. Therefore, in addition to the role of providing strength and stiffness to the anode, the anode support component 5 also functions as an electrolyte creepage barrier, retarding electrolyte creepage from the electrolyte filled anode electrode 2 to the reforming catalyst 11.
In order to perform the barrier function, the openings in the support member 5 should be much larger than the pore size of the anode electrode 2 (for a slower capillary electrolyte transfer) with sufficient thickness (for a longer electrolyte travelling distance). The material of the support member should also be non-wettable to molten electrolyte (i.e., high contact angle) to reduce electrolyte creepage rate. The support also should allow gas access to the anode electrode 2 for the fuel cell reaction. Therefore, the geometry and pattern of the anode support needs to be designed to allow such gas access.
Conventionally, the anode support member 5 may be a perforated Ni plate as disclosed in the ""948 patent (available from Harrington and King and from Ferguson Perforating and Wire). Other conventional forms for the member 5 are expanded Ni mesh (available from Exmet Corporation) and Ni wire mesh (available from Unique Wire Weaving of Hillside, Cleveland Wire Cloth, and Gerard Daniel Worldwide). These forms of the member exhibit low wettability of the electrolyte on the Ni surface.
Use of a perforated Ni plate for the anode support member 2 has certain disadvantages. One disadvantage is the limited manufacturing capability available for forming a thin sheet metal plate with small openings of the size required for the support 5. A second disadvantage is the shadow (blockage) effect caused by the plate for gas diffusion (gas availability to the anode electrode at the non-perforated region). A final disadvantage is the high cost of the plate.
For the mesh type anode support (expanded or wire), the need to partially embed the support into the anode electrode causes a non-uniform thickness of the anode assembly. This results in a non-uniform contact with the anode current collector which, in turn, adversely affects the performance and life of the fuel cell assembly. In addition, the mesh support is also expensive.
It is therefore an object of the present invention to provide a method for fabricating an anode support and a resultant anode support which do not suffer from the above disadvantages.
It is a further object of the present invention to provide a method of fabricating an anode support and an anode support which lend themselves to easier implementation and are less costly.
It is still a further object of the present invention to provide a method of fabricating an anode support and an anode support which promote gas diffusion to the anode electrode and uniform contact with the anode current collector.
In accordance with the principles of the present invention, the above and other objectives are realized in a method for fabricating an anode support in which metallic nickel powder of a predetermined particle size is formed into a powder bed and the powder bed heated at a temperature to cause sintering of the particles. This results in an interconnected sintered nickel porous plaque suitable for use as an anode support.
Preferably, the particle size of the nickel powder is in a range of 45 xcexcm to 100 xcexcm. Also, preferably, the sintering temperature is in a range of 1000xc2x0 C. to 1100xc2x0 C. and the sintering time is in a range of 30 to 120 minutes. Additionally, the resultant porous plaque preferably has a thickness in the range of 250 xcexcm to 400 xcexcm and a porosity of 50% to 65%.
In the embodiment to be disclosed hereinafter, the heating for sintering also is in a protective atmosphere of H2xe2x80x94N2. Also disclosed is an anode assembly formed from the anode support and an anode electrode.